Last week, the EPA Clean Air Science Advisory Committee provided EPA its review of EPA's Policy Assessment for the Reconsideration of the National Ambient Air Quality Standard for ozone.  As expected, CASAC has disagreed with the recommendation of EPA technical staff to retain the current 70 ppb standard.  Instead, CASAC recommends a significantly lower ozone NAAQS of 55-60 ppb. 

This seems to be coming down to a fight between EPA's reliance on controlled human exposure (CHE) studies and CASAC's position that EPA is giving too little weight to epidemiological studies, which show effects at lower levels.

Personally, I'm with CASAC.  Just because it can be difficult to distinguish correlation from causation doesn't mean that the epidemiology shouldn't be given significant weight.  Moreover, as far as I can see, the data seem to be trending in one direction – that ozone causes adverse impacts at lower levels than previously thought.

However, from a legal perspective, I think EPA would probably win a legal challenge to an EPA decision to maintain the 70 ppb standard.  Is the D.C. Circuit Court of Appeals really going to wade into a dispute among scientists between the use of CHE and reliance on epidemiology?  The Court has certainly chastised EPA in the past for ignoring CASAC.  However, this seems a legitimate scientific dispute and while I (and the Court) may think that EPA is wrong, it's not obvious that an EPA decision to retain the 70 ppb standard would be so crazy wrong as to meet the legal threshold we know as “arbitrary and capricious.”

This will be an interesting one to watch.